<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<p>
    <input type="text" name="" id="username">
</p>
<p>
    <input type="text" name="" id="age">
</p>
<p>
    <input type="button" name="" value="提交" id="btn">
</p>
<script>
    var btn = document.querySelector('#btn');
    var username = document.querySelector('#username');
    var age = document.querySelector('#age');
    btn.onclick = function () {
        var xhr = new XMLHttpRequest();

        var nameValue = username.value;
        var ageValue = age.value;
        var params = 'username='+nameValue+'&age='+ageValue;
        xhr.open('post','http://localhost:3000/post');
        xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded')
        xhr.send(params);
        xhr.onload = function () {
            console.log(JSON.parse(xhr.responseText));
        }
    }
</script>
</body>
</html>